Abstract

All organisms have a common ancestor from which all the Kingdoms of Live have diverged. That is why a large number of genes are shared by all living organisms, whereas many others are unique to some specific lineages.

Thanks to the increasing number of genomes, we can estimate which ones are novel and when they appeared in the species evolution.

Thirty years ago, Dr. Doolittle and his team observed that 'some of the most ancient proteins are changing very slowly' (Doolittle et al. 1986). A recent study has verified an 'inverse relationship between evolutionary rate and age of mammalian genes', where they compared orthologous human – mouse sequences (Alba and Castresana 2005).

In this study we show this phenomenon comparing orthologous fugu zebrafish proteins. We classify fugu sequences into three different age groups: eukaryotes, teleosteis and vertebrates.

It has been demonstrated that older genes evolved more slowly than newer ones, so, proteins that arose earlier in evolution currently have a larger proportion of sites subjected to negative selection.

To explain this relationship, we assume that genes, along all their evolutionary history, maintain their functional constrains, but these are much weaker as more recent is the origin of the gene, and selective pressures increase gradually with time (Alba and Castresana 2005) .

In this study it has been observed that the most ancient genes are more conservated whereas the recent groups have a higher divergence. This differences have been verified with a Kolmogorov-Smirnov two samples test.